Hemodynamic monitoring of hospital patients requires that the zero (air) port of the monitoring transducer assembly be at the same level as the right atrium of the patient's heart.
For example, a SWAN-GANZ.RTM. catheter is used to monitor pulmonary artery pressures in patients suffering from massive shock or trauma, various heart conditions, cirrhosis of the liver, or pertonitis, and patients undergoing extensive surgery. Such catheters have a major lumen for measuring pulmonary artery pressures and a small lumen for inflating and deflating the catheter balloon with air or CO.sub.2. In addition, there may be a third lumen which receives a thermistor wire for determining blood temperature and a fourth lumen to monitor the right atrial (central venous) pressure.
The patient normally lies horizontal, as shown in FIG. 1, or in a semi-Fowler's position. Next to the patient's bed is an IV stand having a tubular vertical post 10 on which is adjustably mounted a unitary assembly which includes a strain gauge transducer 11 and a sterile transducer dome 12 directly above it. In the apparatus shown in FIG. 1, two transducer arms 13 and 14 extend up from the dome, one (13) inclined toward the patient and the other (14) vertical. Stopcocks 15 and 16 are located on the transducer arms and an "intraflow" 17 is connected between the stopcock 15 on the transducer arm 13 pointing toward the patient and a flexible tube 18 leading to the patient. The transducer assembly is slidably adjustable along the post 10 of the IV stand, and it has a manually operable clamping arrangement (not shown) for clamping it to the post at whatever height to which it is adjusted.
Stopcock 15 has an upwardly inclined, hollow, arm of cylindrical cross-section which is closed by a removable end cap 15a. When this cap is removed the transducer dome 12 is vented to the atmosphere through this hollow arm of stopcock 15. The other stopcock 16 has a similar hollow arm which, as shown in FIG. 1, extends horizontally and is closed by a removable end cap 16a. When end cap 16a is removed, the transducer dome 12 is vented to the atmosphere through this hollow arm of stopcock 16. Therefore, either stopcock 15 or 16 may be used to provide the zero port for calibrating the transducer 11. This zero port should be at the level of the right atrium, called the "reference level", which is the fourth sternal intracostal space, mid axially. As stated in "Monitoring Pulmonary Artery Pressures" by Susan L. Woods in The American Journal of Nursing, November 1976, Vol. 76, No. 11, the preferred reference level, suitable for persons of any build and in various positions, is a horizontal line drawn through the phelbostatic axis, which is the junction between a transverse plane at the fourth sternal intercostal space and a frontal plane midway between the anterior and posterior chest surfaces.
Prior to the present invention, the usual practice has been for the nurse to locate and mark the reference level on the patient's chest, after which an elongated carpenter's level is held horizontally between this mark on the patient's chest and the IV stand. The unitary assembly of the transducer, dome, transducer arms, stopcocks and intraflow is adjusted vertically along this stand until it is the same height as the reference level, as shown by the carpenter's level extending between it and the patient. This is inconvenient for the nurse and it can be upsetting to the patient, who often is at least partly conscious.